In recent years, a high-level modulation signal such as a quadrature phase-shift keying (QPSK) signal or a quadrature amplitude modulation (QAM) signal is extensively used to increase a transmission capacity per optical fiber.
In order to efficiently manage a next-generation optical network in which the high-level modulation signal is widely used, there is a need to directly monitor a quality of an optical signal in an optical layer. To this end, Korean Registered Patent Publication No. 10-0553814 (published on Feb. 14, 2006) discloses a method for checking the distortion of an optical signal based on a data distribution change caused by a reference voltage variation and a method for monitoring an optical signal-to-noise ratio Q of an optical signal using a non-synchronized amplitude histogram.
Such a conventional method can be realized without having to use a clock signal for synchronization and a high-speed sampling circuit. Thus, the conventional method is economical and applicable regardless of a transmission speed of an optical signal. Recently, it was reported that the conventional method can be applied to a high-level modulation signal such as a QPSK signal, a 16QAM signal or a 6QAM signal. Use of the conventional method makes it possible to monitor not only an optical signal-to-noise ratio but also failure factors of a transmission link, such as a chromatic dispersion (CD) and a polarization-mode dispersion (PMD).
However, an optical signal is asynchronously sampled in the method that makes use of a non-synchronized amplitude histogram. For that reason, the sampled data contains no timing information. Therefore, the signal samples differing in amplitude overlap with each other. This poses a problem in that it is difficult to analyze the non-synchronized amplitude histogram and to extract a suitable parameter indicating a signal quality.
If chromatic dispersions are accumulated in an optical signal, constructive interference and destructive interference occur between continuous symbols. Thus, an increased number of peaks are generated in the non-synchronized amplitude histogram. This makes it difficult to analyze the non-synchronized amplitude histogram.
The method of monitoring a quality of an optical signal using a non-synchronized amplitude histogram is capable of monitoring an optical signal-to-noise ratio and different system failure factors such as a chromatic dispersion and a polarization-mode dispersion. In order to analyze the non-synchronized amplitude histogram, however, the method is applicable only when the optical signal-to-noise ratio is equal to or higher than about 20 dB.
Since data are acquired even in a region where symbol conversion occurs, the distribution of the non-synchronized amplitude histogram varies depending on the rise time of a system and the optical filter existing on a transmission link. Accordingly, the monitoring accuracy is also affected by this factor.
In contrast, if a synchronized amplitude histogram is used, data are acquired at the center of a symbol. This makes it possible to significantly increase the monitoring accuracy. However, a high-speed sampling circuit and a clock-extracting circuit need to be used in order to obtain the synchronized amplitude histogram. For that reason, this method is costly and is not widely used in the art.